When a user installs and runs an application that uses a WebView to embed web content, the WebView will add the X-Requested-With header on every request sent to servers, with a value of the application APK name. It is then left to the receiving web server to determine if and how to use this information.
We want to protect the user's privacy by only sending this header on requests if the app developer explicitly opts in to share with services embedded within the WebView. To achieve this and let current online services that depend on this header migrate away from using it, we will run a Deprecation Origin Trial, while removing the header for general traffic. In parallel, we will be developing new privacy-preserving APIs (such as client attestation APIs) to match the use cases where the X-Requested-With header is being used today.
You can read more about why we're making this change and how it works over at the Android Developer's Blog in Improving user privacy by requiring opt-in to send X-Requested-With header from WebView.